1. Field of the Invention
The present invention relates to a fuel injection control system for an internal combustion engine. More specifically, the invention relates to a fuel injection control with fuel cut-off control in a high engine speed range for preventing discomfortable shock caused due to variation of torque upon fuel resumption and for preventing a catalytic converter in an exhaust system from over-heating.
2. Description of the Background Art
It is well known technique for performing fuel cut-off operation at high engine speed range for preventing the engine from being further accelerated to be damaged due to excessive speed. For example, the Japanese Patent First (unexamined) Publication (Tokkai) Showa 61-55323 discloses a fuel cut-off technique for protecting the engine from being driven at excessively high speed over a revolution limit. In the disclosed system, engine cylinders are divided into two groups. i.e. first and second groups for performing fuel cut-off operation at two stages. Namely, when the engine speed is relatively low, fuel cut-off operation is performed only for the first group of engine cylinders. On the other hand, when the engine speed is substantially high. fuel cut-off is performed for all of the engine cylinders. Such system is effective in reducing shock due to output torque flactuation upon fuel resumption and to reduce magnitude of hunting is engine speed.
Though the system disclosed above can avoid significant shock upon fuel recovery or resumption, it still cause smaller magnitude torque fluctuation to cause still noticable shock. Such shock will degrade riding comfort and drivability especially at the engine driving condition where fuel cut-off and resumption are repeated.
Furthermore, in the condition where the fuel cut-off is performed for only first group of engine cylinders, relatively rich air/fuel mixture is introduced to the second group of engine cylinder. Therefore, larger amount of HC and CO is generated in the second group of cylinders. The HC and CO contained in the exhaust gas from the second group of cylinders causes raising of the temperature of catalytic converter which will cause burn out of the catalytic converter in the worst case.